HELP!! construct an annotated conditional main derivation for the symbolic argument?

Question

1. ~Q->~P
2. P->(Q-> R)
3. Q-> (R-> S)
4. P-> S

Answer 1

assuming line 4 is the conclusion:

1. ~Q → ~P
2. P → (Q → R)
3. Q → (R → S)
4. P ...................assumption for CP
5. Q → R .......... 2,4 mp
6. Q ................. 1,4 modus tolens
7. R .................. 5,6 mp
8. R → S .......... 3,6 mp
9. S .................. 8,7 mp
10. P → S ........ 4-9 conditional proof